A metal material, such as a steel material, is subjected to heat treatments, such as quenching, tempering, annealing, and normalizing, for improving the property thereof. Among the heat treatments, quenching is a treatment for transforming a heated metal material to a prescribed quenched structure by immersing the metal material in a cooling medium, and the quenching makes the treated product very hard. For example, when a heated steel material in an austenite state is cooled at the upper critical cooling rate or higher by immersing in a cooling medium, the material can be transformed to a quenched structure, such as martensite.
As the cooling medium, an oily or aqueous heat treatment agent is generally used. The quenching of a metal material with an oily heat treatment agent (i.e., a heat treatment oil) will be described. In the case where a heated metal material is placed in a heat treatment oil as a cooling medium, the material is generally cooled through three stages. Specifically, the stages include (1) the first stage where the metal material is enclosed with a vapor blanket of the heat treatment oil (vapor blanket stage), (2) the second stage where the vapor blanket is ruptured, and boiling occurs (boiling stage), and (3) the third stage where the temperature of the metal material becomes the boiling point of the heat treatment oil or lower, and the heat is removed through convection (convection stage). The cooling rates in the stages are different from each other due to the difference of the atmosphere surrounding the metal material, and the cooling rate in the second stage (boiling stage) is the largest.
In a heat treatment oil, the cooling rate is generally increased quickly in the transition from the vapor blanket stage to the boiling stage. In the case where the metal material does not have a simple flat shape, the vapor blanket stage and the boiling stage tend to be mixedly present on the surface of the metal material. In the case where the stages are mixedly present, an extremely large temperature difference occurs on the surface of the metal material due to the difference in cooling rate between the vapor blanket stage and the boiling stage. The temperature difference forms temperature stress and transformation stress, which cause distortion of the metal material.
Therefore, in a heat treatment of a metal material, particularly quenching thereof, it is important to select a heat treatment oil that is proper for the heat treatment condition, and if the heat treatment oil is selected improperly, there may be a case where distortion occurs in the metal material, and sufficient quenching hardness cannot be obtained.
The heat treatment oil is classified into a cold oil used at a low oil temperature and a hot oil used at a high oil temperature.
The cold oil generally contains a low viscosity base oil to have a large cooling rate, and thus has high cooling property. However, the cold oil undergoes a long vapor blanket stage, which tends to make the vapor blanket stage and the boiling stage mixedly present on a surface of a metal material, and thus distortion tends to occur. Accordingly, in many case, a vapor blanket breaking agent is blended in the cold oil to shorten the vapor blanket stage.
The hot oil undergoes a short vapor blanket stage and hardly forms distortion, but in recent years, a vapor blanket breaking agent is blended in the hot oil in some cases for further decreasing distortion.
The vapor blanket breaking agent used above includes asphalt, and also includes an α-olefin copolymer (see PTL 1) and an imide compound (see PTL 2).